1. Field of the Invention
This invention relates to a system and method of enhancing vehicle stability including reducing a likelihood of rollover for motor vehicles and particularly for SUV's, and yaw control to reduce/prevent vehicle skidding and spin-outs by providing a pulse active steering control system and further relates to a steering control system and motor vehicle in combination.
2. Description of the Prior Art
With the increase in popularity of Sport Utility Vehicles (SUV's), the total number of deaths and injuries caused by rollover crashes has increased significantly since the 1990's. Some statistics have shown that nineteen (19) percent of rollover accidents involve SUV's, yet SUV's remain popular.
Various control systems are known to reduce the likelihood of rollover in motor vehicles including active front steering control, differential braking control and integrated active steering and differential braking control. In previous steering control systems, simulations have been carried out when driver's steering input is fed to a tire model to calculate the tire forces. The tire forces are then input to the vehicle yaw/roll model to calculate the yaw rate, roll rate, longitudinal and lateral velocities and, finally, all resulting parameters are input into the rollover estimator to calculate the rollover coefficient and its error signal when it goes beyond its designated threshold. A controller such as proportional-derivative (PD) controller then receives the error signal and generates the necessary additional steering angle to compensate for the original driver steering input in order to reduce a likelihood of rollover. A similar control structure where the error signal presents the difference between the estimated and actual measured vehicle yaw rate have been used to reduce or prevent vehicle skidding and spin-outs.
Unfortunately, with previous active front steering control systems, while the rollover coefficient is reduced, the rollover coefficient may not be reduced sufficiently to prevent the vehicle from rolling over. In the above control strategy when the gain of the controller is increased, the system becomes unstable. Vehicle stability issues are of particular concern with road vehicles and rollover is particularly a concern to SUV's. In order to be within the rollover safety range, the rollover coefficient must be between −1 and 1.
Another prior art system that is used for vehicle stability is a differential braking control system. The system operates in a manner similar to the active front steering control system where a PD controller controls the magnitude of the torque required to prevent rollover or skidding by applying different braking forces to the vehicle wheels. The tires on the two outside wheels in a turn are braked by the controller when the rollover coefficient or yaw error reaches the designated threshold. As can be seen from FIG. 5, the differential braking control system is sufficient to reduce the rollover coefficient to a level within the safety range. It has been shown that the efficiency of a differential braking control method can be greatly reduced when the vehicle mass distribution changes significantly due to the number of passengers, luggage or any other item with significant weight.
However, when the differential braking control system is used with simulation model #2, which is a non-linear tire model with the same non-linear yaw/roll model as in model #1, the differential braking system does not reduce the rollover coefficient to a level within the safety range. Model #1 uses a simple linear tire model incorporating the non-linear yaw/roll model. At high vehicle operating speeds, if driver steering inputs are at extreme values, the rollover coefficient exceeds the safety range by an amount that may not be able to be compensated by the differential braking control system. This can result in significant shift in vertical tire load to one of the front tires causing abnormal tire lateral forces. In addition, a vehicle that is loaded with cargo, passengers, or both or even has small amount of cargo or one passenger will pose a greater problem for the differential braking control system which may not be able to reduce the rollover coefficient to a level within the safety range.
It is also known to integrate an active steering system with a differential braking system. In FIG. 8, the rollover coefficient is successfully reduced to a level within the safety range by differential braking control system in combination with an active steering control system using a linear tire model. However, when the identical simulation is applied on a fully non-linear model with a non-linear tire model, the integrated active steering differential braking controller still has difficulty reducing the rollover coefficient entirely within the safety range as is illustrated in FIG. 9.
While the integrated controller has a better potential to reduce rollover risk to a level within the safety zone, when applying the controller to a fully non-linear vehicle model in simulation, the controller still fails to prevent rollover.
In Chen et al US Patent publication 2005/0043874 published Feb. 24, 2005, an active steering control system for a motor vehicle is described.